This is an application for a follow-up study of 50 premenopausal black and white women who will have been reduced to a post-obese state and matched by age, ethnicity and body composition with 50 never obese control women with no family history of obesity. Hypotheses are based on previous data suggesting that the component of energy expenditure most important in the etiology of obesity is activity related energy expenditure (AEE). The investigators propose that abnormal muscle energy metabolism results in low spontaneous AEE in obesity prone individuals and this in turn results in greater long-term weight gain than in the never obese controls. The investigators thus plan to follow 50 post obese and 50 never obese women for four years without intervention. These women are the participants in a previously funded project to study weight loss. At yearly intervals in the CRC the investigators will measure physical activity, body weight and composition, AEE under sedentary and free living conditions by doubly labeled water and by whole-room indirect calorimetry, oxidative capacity during treadmill exercise and by NMR during isometric exercise, oxygen costs during standardized exercise, ATP production rate in exercised muscle, exercise difficulty using measures of physiologic stress and perceived stress, fuel utilization by indirect calorimetry, and insulin sensitivity by the FSIGT.